We consider the variation in miscibility during a polycondensation reaction which forms a random multiblock copolymer. Taking a "Markov chain" model for the reacting polymer system, we apply the random phase approximation and calculate the spinodal as a function of degree of reaction. In order to achieve this, we develop a formalism for the calculation of structure factors of polymers of arbitrary complexity. Taking a model two-component system, we show that phase separation may begin at an early stage in the reaction process, while the chains are still short. We also show that in the earliest stages of reaction, this model system becomes more miscible with extent of reaction. Finally, we generalize the theory to branched Markov systems.